The present invention relates to magnetic bubble memories, and more particularly, to a magnetic bubble memory having an increased storage density and a reduced operating drive field achieved through increasing the gap tolerance between adjacent bubble propagation elements.
Information storage and retrieval in conventional magnetic bubble memory devices is accomplished and measured by the presence or absence of magnetic bubble domains which are propagated and manipulated on a chip. Typically, the chip includes a plurality of spaced apart permalloy propagation elements which overlie a thin film of garnet. The maintenance and propagation of the magnetic bubble domains along the paths of the propagation elements is accomplished through the utilization of an in-plane rotating magnetic field (XY drive field) in the presence of a bias magnetic field (Z bias field.) As the drive field rotates the bubbles jump between adjacent propagation elements crossing the gaps which exist therebetween.
Heretofore the permalloy propagation elements have assumed a wide variety of configurations. In the beginning T-I bar configurations and Y-bar configurations were widely used. These are described in a publication entitled "Magnetic Bubble Technology: Integrated-Circuit Magnetics for Digital Storage and Processing," edited by Hsu Chang, IEEE Press, 1975 Library of Congress catalog No. 73-87653, pages 20-24, beginning with the paragraph entitled "Field Access Devices-Rotating Planar Field."
In the prior mentioned T-I and Y-bar patterns, the minimum feature size, that is the smallest dimension in the pattern, which in these patterns is the gap between the T-I or Y-bars, is approximately 1/3 to 1/2 of the bubble diameter. Since the resolution limit of current photolithography used in the fabrication processes of forming the overlays is somewhere between 1 to 1.5 microns, the smallest bubble diameter that can be used with these patterns is generally about 4.0 microns.
Presently by far the most utilized bubble propagation element configurations are the so-called half-disk or C-bar and the assymmetric chevron. The former configuration is described in an article entitled "Gap Tolerant Bubble Propagation Circuits" written by I. S. Gergis, P. K. George, and P. Kobayashi, and published in the IEEE Transactions on Magnetics, Vol. MAG-12, No. 6, November, 1976 pages 615-653. The latter configuration is described in an article entitled "The Development of Bubble Memory Devices," Electro 77 Paper 12/1, written by A. H. Bobeck.
The half-disk and assymmetric propagation configurations are sometimes referred to as being "gap tolerant." Their main advantage lies in the fact they have a minimum feature size, i.e. the gap between opposing legs of adjacent elements, which is approximately 2/3 of the bubble diameter. Therefore, for a selected bubble size, e.g. 3.0 microns, the gap can be 2.0 microns. The utilization of the half-disk and assymmetric configurations leads to less stringent photolithographic accuracy than the utilization of earlier configurations. In addition with the half-disk and assymmetric chevron configurations, it is possible, by maintaining more stringent photolithographic accuracy, to design a magnetic bubble memory with a smaller bubble size than was previously possible before the introduction of these configurations. The operating drive field is relatively low for small bubbles. A reduction in the magnitude of the operating drive field results in lower power dissipation which results in both power savings and cost savings in the drive field circuitry. A review of various gap tolerant propagation element configurations can be found in a book entitled Magnetic-Bubble Memory Technology, by Hsu Chang, 1978, Marcel Dekker, Inc. Publisher, pages 52-56.
Heretofore, with known propagation element configurations including the half-disk and assymmetric chevron, a short between proximate permalloy elements has created a barrier to bubble propagation. The existence of too many shorts in a given chip has required that the chip be discarded. However, with the half-disk and assymmetric chevron propagation elements small bubbles sizes can be utilized with gap sizes significantly larger than the resolution limit of the photolithography. Therefore such bubble memories can be fabricated with a minimum number of shorts resulting from tolerance variations due to fabrication inaccuracies.
Clearly the bit storage capacity on a bubble memory chip is directly proportional to the gap size. If the gap size can be made smaller, both the number and size of the information storage loops which can be placed within a given chip area can be increased. In summary, it is the size of the gaps between adjacent propagation elements which limits the size of the magnetic bubble domains and thus the storage density of the device.
Heretofore, every known permalloy pattern has incorporated linear arrays of spaced apart propagation elements which have been configured and positioned to present opposing, parallel side edges. Both the half-disk and assymmetric chevron configurations each include a pair of legs. When delineated over the garnet, the legs of adjacent half-disk or chevron elements have been positioned so that their opposing side edges are parallel, the gap therebetween having a uniform width throughout the side dimension of the adjacent elements. Thus, if a short has occurred between adjacent elements it has typically resulted from the legs of adjacent elements overlapping one another. Such a short is referred to herein as a "wide short" since it exists throughout the entire side dimension of the adjacent elements. Such a wide short carries a considerable amount of flux and it effectively destroys the poles of the propagation elements. This creates a barrier and impedes the propagation of the magnetic bubbles down the pathway of elements containing the short. Thus, due to the resolution limit of current photolithography there has been an artificial lower limit on the gap size and thus an artificial upper limit on the storage density of prior magnetic bubble memories. This is especially understandable when one considers that the major portion of the area of a bubble memory chip is typically devoted to information storage loops comprised of either the half-disk or assymmetric chevron propagation elements.
U.S. Pat. No. 4,094,004 discloses a magnetic bubble expander-detector circuit for a magnetic bubble memory. This patent recognizes that gap tolerance between adjacent propagation elements in effect places an artificial lower limit on the size of the magnetic bubbles and an artificial upper limit on the density of the device. However, the patent further states that the elimination of the gaps alone will not suffice to eliminate these artificial limits since it is asserted that this would destroy necessary propagation directionality. The invention disclosed in this patent eliminates the horizontal gap between adjacent chevron propagation elements in the expander-detector pattern of the bubble memory detector circuit. However, a vertical gradient or vertical gap between the parallel side edges of adjacent chevron elements is substituted for the horizontal gap which has been eliminated (see FIG. 3.) This vertical gap is imparted by an underlying silicon dioxide wedge 74 as seen in FIG. 3. Clearly, the disadvantage of this construction is that the formation of precise wedges would be difficult to say nothing of the possible deleterious effects that might result from having propagation elements inclined with respect to the drive and bias fields.